Bee colonies are generally kept by beekeepers in beehives, also called brood boxes, and are used here to produce honey. The beehives are normally cuboid. Rectangular middle walls, suspended next to one another, are inserted into the beehives and are surrounded by frames by means of which they can be inserted removeably into the beehives. Within the context of the present description and claims the term “ middle wall” is to be understood here as meaning a wall with any outline that is provided on one or on both sides with pre-shaped brood cells independently of whether the middle wall is surrounded by a frame or not.
In most cases the middle walls are in the form of rolled or cast sheets made of bees wax, in which the brood cells are impressed in the form of hexagons with equal sides and which respectively form the honeycombs. After suspending the middle walls in the bee colony, the bees grip onto the honeycomb pattern and extend the pre-formed brood cells with the aid of the bees wax produced by them so as to form a brood comb.
As regards the middle walls, a distinction is made between middle walls with brood cells for female bees—the so-called worker inner cells, and brood cells for male bees—the so-called drone cells. The latter have a considerably greater diameter and depth than the worker inner cells.
For a number of years beekeepers have been confronted with a drastic increase in the mortality rate of bee colonies, in particular winter bee colonies. Mites that are harmful to bees, especially the varroa mite, are held responsible for the high mortality rate. In order to combat the varroa mite, basically two methods have been used, namely on the one hand chemical treatment, for example by means of organic acids, and on the other hand thermal treatment. The disadvantage of the chemical treatment is that there is the risk that residues from the treatment product will pass into the honey. This risk does not exist with the purely thermal treatment.
For the thermal treatment special heating cabinets have been developed in which the brood combs are subjected to a flow of warm air for a number of hours. For this purpose the brood combs infested with varroa mites are removed from the beehive, and the bees sitting on the latter are shaken or wiped off. The bee-free brood combs are then suspended in the heating cabinet. Then the temperature of the flow of warm air is slowly increased until a temperature of between 39 and 42° C. is reached in the brood cells. This temperature is then maintained for a number of hours (WO 92/14355 A1 and EP 2 250 880 B1).
The treatment by means of a separate heating cabinet is very time-consuming due the necessity of bringing the brood combs into the heating cabinet and back into the beehive. Moreover, the purchase of the heating cabinet represents a considerable, hardly cost-effective investment for amateur beekeepers. Therefore, due to the simpler handling and the lower costs, many beekeepers therefore still resort to the chemical approach.
As an alternative to the warm air treatment in separate heating cabinets it has been proposed to combat the varroa mite by providing a middle wall or a number of middle walls with brood cells for the drones with a heating device that has a resistance heating element embedded in the middle wall and a control device connected to the latter (see U.S. Pat. No. 8,272,921 B1). The middle wall equipped in this way or the middle walls equipped in this way are inserted into the brood box and is/are extended by the bees to form a drone brood comb or drone bee combs. During the development of the drone larvae into adult drones the heating device can be switched on in order to heat the drone brood comb or the drone brood combs. In a few minutes this causes the drone brood comb(s) to heat up to approximately 65° C. After a similarly small number of minutes the heating device is switched off again. This strong heating causes both the varroa mites and the drone brood to die.
It is an advantage of this system that no special heating cabinet with handling necessary for its use is required. It is a disadvantage, however, that the effectiveness is unsatisfactory because the combating of the varroa mites is restricted to the drone brood comb designed for drones, and accordingly the brood cells for the workers, which are substantially more important for the bee colony population, remain untreated, and so unprotected. In addition, that brood combs for the drone brood are only liable to a varroa mite infestation during the relatively short breeding season for drones, and so it is only possible to combat the varroa mite during this time.